1. Field of the Invention
The present invention generally relates to an optical writing unit, a driving method thereof, and an image forming apparatus that are applicable to digital output equipment, such as a digital copier, a printer, and a digital FAX. The present invention specifically relates to the optical writing unit, the driving method thereof, and the image forming apparatus consisting of a light emitting device array consisting of a plurality of light emitting device array chips, each chip consisting of a plurality of light emitting devices that are arranged at a predetermined interval, and an image forming device array.
2. Description of the Related Art
In recent years and continuing, miniaturization of the optical writing unit for performing digital writing is required as the miniaturization of digital image output equipment, such as a digital copier, a printer, and a digital facsimile, progresses.
Methods of digital writing can be broadly divided into two kinds at present.
One of the methods is an optical scanning method wherein optical scanning is carried out by a light flux irradiated from a luminous source, such as semiconductor laser, which is deflected by an optical deflector, and an optical spot is formed through a scanning image-forming lens. The other method is a solid-state optical writing method wherein an optical spot is formed by a light flux irradiated from a light emitting device array, such as a light emitting diode (LED) array and an organic EL (electroluminescent) array, the light flux being put through an image forming device array.
The optical scanning method, using the optical deflector for scanning, tends to have a long optical path. On the other hand, the solid-state optical writing method requires a short optical path, and therefore, can be compactly structured. Further, the solid-state optical writing method does not use movable parts like an optical deflector; therefore, there is an advantage that noise can be suppressed (low noise). The optical writing unit using the solid-state optical writing method consists of a light emitting device array that consists of a plurality of light emitting devices, and an image forming device array that consists of a plurality of image forming devices. Here, a conventional example of the optical writing unit using a rod lens array is described with reference to FIG. 40.
The optical writing unit shown in FIG. 40 includes a light emitting device array chip 324, consisting of a plurality of light emitting devices arranged at a predetermined interval on a substrate 323; the substrate 323 is contained in a container indicated by 321 and 325; and an image forming device array 322, consisting of a rod lens array for forming an optical spot by light irradiated from each light emitting device, the optical spot being irradiated on a photo conductor 326 that is an image supporting object for image formation. As the light emitting device array, a light emitting diode array, wherein light emitting diodes are arranged at a predetermined interval, for example, is widely used. The light emitting diode array includes dozens to hundreds of light emitting diode-array chips mounted on the substrate, each of the diode-array chips including dozens to hundreds of light emitting diodes arranged at the predetermined interval. Here, the light emitting diode-array chips are mounted on the substrate such that the interval of the light emitting diodes on the edges of adjacent light emitting diode-array chips desirably coincides with the predetermined interval.
FIG. 35 through FIG. 37 show an outline structure of the light emitting diode array. FIG. 35 is a plan drawing. FIG. 36 is a cross-sectional drawing. FIG. 37 is a plan drawing of the light emitting diode-array chip 324. As shown in FIG. 35 through FIG. 37, a plurality of light emitting diode-array chips 324 are mounted on the substrate 323. As shown in FIG. 37, a plurality of light emitting diodes 328 are mounted in each of the light emitting diode-array chips 324. Adjacent to each of the light emitting diode-array chips 324, driver ICs 329 and 330 are mounted on the substrate 323. The light emitting diode 328 of each light emitting diode-array chip 324 is driven by the driver ICs 329 and 330. Further, a connector 327 for connecting signal lines for sending luminescence data is formed. Further, as for the image forming device array 322 as shown by FIG. 40, a rod lens array consisting of a plurality of refractive-index distribution type rod lenses, which are bundled, are used. As shown in FIG. 38, the rod lens array is structured by bundling two sequences of the rod lenses 322a and 322b such that cross-sectional centers of the rod lenses form the apexes of equilateral triangles. Side plates 331a and 331b maintain the circumference of the rod lens array. Gaps between the rod lenses 322 are filled with an opaque material 334, which is solidified.
Other examples of the image forming device array are a roof prism lens array (RPLA), as shown in FIG. 39, and the like. In the RPLA, an incidence side lens face 335a, an ejecting side lens face 335b, a rib 336, and a roof prism 337 are formed in one body, as shown in FIG. 39. Further, an optical attenuator 339 of an opaque component for interrupting ghost light may additionally be provided between a set of lenses 338a and 338b, and a set of lenses 338a′ and 338b′, as shown in FIG. 41.
Now, the interval between any two adjacent light emitting diodes is desired to be uniform in the entire light emitting diode array. The uniformity in the interval is usually obtained within a light emitting diode array chip. However, it is difficult to maintain the uniformity in the interval between a light emitting diode on the edge of a light emitting diode-array chip 324 (chip) and another light emitting diode on the adjacent edge of the adjacent chip. This is because of the difficulty in maintaining uniformity in the distance (gap) between the light emitting diode-array chips 324 for manufacturing reasons.
Specifically, it is difficult to accurately control the distance (gap) between the light emitting diode-array chips (the chips) when mounting the chips on the substrate. If the light emitting diodes that are adjacent across the gap between the chips are apart too much, a crevice in an image is generated on the surface of the image bearing object 326 (drum, photo conductor), resulting in a white stripe on a printed image. If, conversely, the light emitting diodes that are adjacent across the gap between the chips are positioned too close to each other, optical spots generated by the light emitting diodes overlap, resulting in a black stripe on a printed image. For this reason, a conventional problem is that the chips have to be re-mounted on the substrate.
Then, a circuit for compensating for the crevice and the overlap has been considered, wherein an additional transistor is provided to the light emitting diode on the edge of the light emitting diode-array chip for adjusting the driving current of the light emitting diode on the edge of the light emitting diode-array chip, and an additional circuit is provided for compensating for the illuminating period. However, these solutions make the driving circuit complicated. It is far more desirable that the light emitting diodes be uniformly placed at a predetermined interval in the entire effective image domain. The interval of the light emitting diodes is explained with reference to FIG. 1 that is also used in explaining the embodiments of the present invention. As shown in FIG. 1, a plurality of light emitting diode-array chips 2 are mounted on a substrate 1, and a plurality of light emitting diodes 3 are mounted on each of the light emitting diode-array chips 2. It is desirable that all the light emitting diodes 3 be mounted at the predetermined interval P. However, in fact, the mounting interval of the light emitting diode-array chips 2 cannot be made uniform on the substrate 1, due to mounting errors.
That is, an interval Pa between light emitting diode 3 on the edge of a light emitting diode-array chip 2 (chip) and another light emitting diode 3 on the edge of an adjacent chip becomes different from the predetermined interval P. When the substrate 1, having non-uniformity in the intervals between the light emitting diodes, is used by an optical writing unit of an image forming apparatus, a resulting image output bears a black vertical line and a white vertical line, degrading the image quality. In view of the problem, JP,8-118722,A discloses a solution.
In the Japanese patent application, adjustment means are provided for adjusting the current supplied to the light emitting diodes on the edges of each light emitting diode-array chip. The means is provided in the driving circuit that supplies the driving current to each of the light emitting diodes. The driving circuit is included in a light emitting diode printing head, which also includes a plurality of the light emitting diode-array chips mounted on a substrate at a predetermined interval in the shape of a straight line. Each of the light emitting diode-array chips includes a plurality of light emitting diodes.
By adjusting the driving current of the light emitting diodes with the adjustment means, the light volume of the light emitting diodes on the edge of the light emitting diode-array chips can be increased or decreased. When the distance between the light emitting diodes on the edges of the adjacent light emitting diode array chips is too great, and a crevice is generated in the irradiation region of the drum, the luminous intensity of the light emitting diodes is increased by the adjustment means, thereby the white stripe is not generated in the printing result. However, in the case of JP,8-118722,A, the adjustment means is needed, and circuit arrangement becomes complicated as the result.
Further, according to the above-described means, only the luminous intensity adjustment of only the light emitting diodes on the edges of the light emitting diode-array chips is performed, which is insufficient as explained below using FIG. 4 that is also used for explaining the embodiment of the present invention. Namely, if Pa is greater than P, the above-described means increases-the light volume of the light emitting diodes in two vertical columns indicated by “a”, in reference to FIG. 4, where a line C indicates the junction position of the two light emitting diode array chips, that are adjacent to each other, thereby the image dots D in the two vertical columns “a” are enlarged. The enlarged image dots may suppress the appearance of a white stripe at the junction position C of the two adjacent light emitting diode-array chips. However, since the image dots D of the vertical columns “a” are set greater, a black stripe may now appear at intervals marked by “b” between two columns of the light emitting diodes, one of the two columns being the column “a”, and the other being the adjacent column within the same chip. For this reason, it is likely that the adjustment of only the light volume of the light emitting diodes only on the edges of the adjacent light emitting diode-array chips is insufficient to make the-white stripe and the black stripe inconspicuous.
As described above, according to the conventional technology, when a white stripe and black stripe arise in the printing result due to non-uniformity in the distance between the light emitting diodes on the edges of the light emitting diode-array chips, it is necessary to carry out one or any combination of re-mounting the light emitting diode-array chips on the substrate, providing the adjustment means for compensating for the differences in the distances of the light emitting diode-array chips, and providing other means, which complicates the circuit arrangement.